The growing demand for freeze-dried products has led to ever increasing demands for more efficient means of performing all of the operations necessary to obtain a freeze-dried product. A critical step in the process is the actual freezing of the product to be dried. Generally, it is recognized that rapid freezing will promote the growth of a large number of well-ordered dendritic ice crystalls of small size and that slow freezing will produce a lesser number of non-ordered dendritic ice crystals of a larger size. Thus, in the dehydration of solid foods, the cellular structure of the food can be modified by varying the freezing rate. Since liquid food material, such as coffee extract, lacks the cellular structure of solid foods, various methods have been advanced to promote the formation of dendritic ice crystals during the freezing process of the extract. Large non-ordered dendritic ice structures, when sublimed, permit a product which is darker and more coffee-like in color than a product based on the smaller structures.
One method is dynamic freezing of the extract. Here the extract is agitated or vibrated to prevent supercooling and spontaneous nucleation. Heat removal from the extract is thus more uniform. Another method is static freezing in which no overt agitation is used and the liquid material is slowly frozen by contact with a cooling surface which allows slow heat removal from the body of the extract. Still another method is to seed the extract with previously formed ice crystals. These methods, however, have inherent disadvantages or limitations. Agitation is only feasible during the early stages of chilldown while the extract is still liquid. Likewise, seeding techniques are performed while the extract is still in a liquid state. In static freezing, the time needed to permit the growth of large dendritic ice crystals necessarily limits the overall product output rate.
The freezing of coffee extract in preparation for freeze-drying is commonly done on a continuous stainless steel or teflon coated belt. Liquid slushed or foamed extract is normally fed onto one end of the belt by a pipe or spreader device. Additional means such as a dam or partitions may be employed to insure that the extract uniformly covers the width of the belt. Freezing or heat removal is by conduction through the belt, such as by continually spraying the underside of the belt freezing surface with brine. Commonly, a series of brine tanks are maintained at progressively lower temperatures from the beginning of the belt to the end. A typical temperature range is from about plus 10.degree.F to minus 40.degree.F or below. The temperature profile across the length of the belt is dependent on the freezing rate characteristics desired. The freezing rate for coffee extract is a major factor in the final product's color. Maintaining all of the brine at its lowest possible temperature results in a very high freezing rate. However, such high freezing rates produce a product which is light brown or tan in color. As the freezing rate is slowed, a darker, more desirable product is obtained. However, this slow freezing by temperature profiling limits productivity.
The extract applied to the belt, as above described, spreads out as a relatively uniform layer and is frozen into a solid slab, usually about 1/4 to 1 inch in thickness. Various methods of freezing the coffee extract, all of which require an extended period of time, are illustrated in U.S. Pat. Nos. 3,253,420 to DeGeorge, 3,399,061 to Lutz, 3,443,963 to Simon et al., and 3,809,766 to Stolz.
It would, therefore, be highly desirable if the simple method would be devised for freezing coffee extract at a relatively high rate, while at the same time, promoting the growth of large ice crystals whereby the finished product is dark and coffee-like in color.
As used in this application, "Extract" means an aqueous extract of coffee solids having a concentration of between 15 and 50 percent soluble solids by weight. "Icepoint" is that temperature at which water in the extract begins to crystallize into water ice. "Eutectic point" means that temperature at which the specific mixture of coffee solids volatile aromatics and water having the lowest melting point of any other mixture in the extracts solidifies. In effect, this temperature is the lowest possible melting point of any material contained in the extract. "Product temperature" as used herein means the temperature of the dried coffee extract. "Freeze-drying" as used in this context refers to the process of drying whereby water is removed directly from the solid state to the vapor state without passing through an intermediate liquid state (sublimination). This process also includes that portion of the drying process wherein all the water ice crystals have been sublimed and a eutectic mixture of coffee solids aromatics and water is dried to a stable moisture content. In this portion of the freeze-drying process, it is possible that some evaporation of water from the liquid state may occur without appreciable melting of the frozen extract. However, even at this stage of the drying, the product temperature should be kept below the eutectic point of the material being dried in order to avoid melting any portion of the frozen coffee extract.